This invention relates generally to ablative liner materials and, more particularly, to polymeric ablative liner materials for rocket combustion chambers and expansion nozzles.
Rocket combustion chambers and expansion nozzles, especially those used in liquid oxygen-hydrogen propulsion systems, must withstand high temperature gases which sometimes exceed 3000 degrees C. Three general approaches are typically used for protecting the inner walls of these propulsion systems. The first approach is regenerative cooling, a process in which the liquid propellant provides cooling for the walls of the combustion chamber and expansion nozzle. The second approach utilizes a machined metal liner of some exotic refractory metal, such as columbium, to protect the inner walls of the combustion chamber and expansion nozzle. However, both of these approaches are very costly.
The third approach is the use of an ablative liner. One common type of ablative liner is a machined composite liner, such as a silica phenolic composite, which fits inside the combustion chamber and expansion nozzle. However, machined composite liners require numerous fabrication steps and are subject to severe cracking because of their stiffness. Another common type of ablative liner is a semi-liquid formable polymeric liner which is applied to the inner walls and cured in place. This is the most desirable type of ablative liner because it is low cost, easy to fabricate and resists severe cracking.
Many types of polymeric ablative liners are available, but those based on silicone polymers are particularly attractive. However, none of the presently available polymeric ablative liners has been tailored to optimize the ablation process for liquid-propellant rocket engines. Ablation involves an endothermic chemical reaction in which the liner material is thermally degraded in a controlled manner to produce gases and a porous residue or char of glasses and carbon having a low thermal conductivity. The heat required to sustain the endothermic chemical reaction and the generation of gases provides the cooling. The residue or char that remains after the chemical reaction is completed also provides an insulating barrier for protecting the combustion chamber and expansion nozzle during the latter stages of combustion. These properties of the endothermic chemical reaction, the amount and type of gas generated by the chemical reaction and the amount and stability of the char, must be optimized to provide the greatest degree of protection for liquid-propellant combustion chambers and expansion nozzles. The present invention is directed to this end.